This invention relates to a diffusion apparatus. It relates particularly to a diffusion apparatus for use in the extraction of sugar cane juice from "prepared" sugar cane.
Diffusion is an alternative to heavy duty milling as a method of extracting sugar cane juice from prepared sugar cane. As in the milling process, in the diffusion process "imbibition" water is added to aid extraction of the cane juice and essentially flows counter-current to the cane fiber to optimize the extraction of the sugar juice from the cane fibers.
Prior to either the milling or diffusion-extraction processes, sugar cane is typically "prepared" by knifing and/or shredding the cane stalks to expose the juice cells. In this regard, the term "preparation index" (P.I.) is often used in the sugar cane industry to describe a scale that defines the extent to which the sugar cane is knifed and/or shredded, with a high preparation index being indicative of a high degree of opened cells.
The process of diffusion used in the extraction of sugar juice from sugar cane is well-known and involves repeatedly washing "prepared" sugar cane with hot imbibition water.
The extraction of sugar cane juice in a diffusion apparatus occurs partly by the physical action of repeated washing and draining of the water by and from the prepared sugar cane, and partly due to the process of osmosis.
Diffusers are designed to extract sugar cane juice either from prepared cane (in which case it is called a "cane" diffuser) or from prepared cane that has been passed once through a sugar mill (in which case it is called a "bagasse" diffuser). For the purpose of this patent application there will be no differentiation between cane and bagasse diffusion.
The most successful known sugar cane diffuser in use is the "Egyptian" diffuser (also referred to as a "breadbox" diffuser). This type of diffuser is commonly used in the South African sugar industry which has the highest sucrose-extraction level of any sugar cane industry in the world, largely due to its having converted from milling to diffusion-extraction at most of its sugar factories.
Despite the high extraction levels obtained, the Egyptian diffuser has a number of disadvantages:
a) Egyptian diffusers are large in size (typically 65 meters in length) and therefore costly to construct. The width of the diffuser is proportional to the required throughput thereof and, as such, is generally between 4 and 12 meters wide. PA1 b) In an Egyptian diffuser, the prepared cane is slowly moved over a perforated deck at a depth of typically 1.4 to 1.8 meters where it is repeatedly exposed to the washing action of the imbibition water. As such, an Egyptian diffuser has multiple stage-pumps, piping and distributors to recirculate the water/juice mixture which percolates through the bed of prepared cane. PA1 c) If an Egyptian diffuser is installed in an existing factory, it is typically situated some distance from the initial preparation equipment and final processing equipment and invariably relatively lengthy and expensive conveyors are required to feed the diffuser and return the residual fiber. PA1 d) If the preparation index of the cane is relatively high (typically&gt;93%), the fines in the cane fiber have a tendency to "blind" the blanket of sugar cane, thereby inhibiting percolation and causing flooding on the surface of the cane bed which results in poor extraction. This is typically catered for by installing lifting screws to disturb the bed of cane at two or three places along the length of the diffuser. However, these lifting screws can be very troublesome and have been known to cause many stoppages. PA1 e) Egyptian diffusers require regular monitoring to ensure optimum operation. PA1 f) Egyptian diffusers and also other known diffusers require relatively high imbibition rates (typically 350 to 400% of the fiber flow rate). This in turn requires increased evaporation capacity in the final processing stage and increases steam demand from boilers. This can have serious financial implications when installing in an existing factory that typically uses low imbibition levels (100-200% on fiber). PA1 a diffusion tank for containing imbibition water, having an operative front end, an operative rear end, two spaced sides extending between said front and rear ends and a base, the diffusion tank including a plurality of spaced operative upright partition members located between said front and rear ends of the diffusion tank, extending between the sides thereof, each partition member defining an operative upper and lower end, with the lower end thereof being spaced from the base of the diffusion tank; and PA1 conveying means for conveying prepared cane from the front end of the diffusion tank to the rear end thereof along a continuous serpentine-like path wherein the prepared cane is conveyed along spaces defined between each of adjacent partition members located in the diffusion tank.
Any reference hereinafter to sugar cane must be interpreted to include sugar beet. Furthermore, any reference hereinafter to prepared cane must be interpreted to mean, in the case of sugar cane, sugar cane that has been prepared by knifing and/or shredding sugar cane stalks, and in the case of sugar beet, sliced sugar beet.
It is an object of this invention to provide a diffusion apparatus that eliminates or avoids the disadvantages of the Egyptian Diffuser.